That is indeed the Williams Bros. kit # 32526 Gee Bee Z Super Sportster.
It's currently available from Williams Bros., so it's not particularly collectable. From the looks of the decal sheet (good), yours is probably ten years old or less.
This one is an attractive scheme, a departure from the more usual red/ white.
Those exploded instructions shown are a WB give away, as are the individual engine pots.
WB is a great company, and if you contact them I'm sure they'll set you up with the plans.
And it's 1/32 scale, still a very small airplane.